Biological nitrification plays a key role in the global nitrogen cycle, which is considered to consist of two serial steps of ammonia oxidation to nitrite biocatalyzed by ammonia oxidizing microorganisms (AOM) and nitrite to nitrate biocatalyzed by nitrite oxidizing bacteria (NOB). AOM include ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA), which are widely spread with relative abundances closely related to the concentration of ammonia. In 2015, three bacteria belonging to Nitrospira lineage II with NOB-specific functional genes were identified to carry AOM-specific functional enzymes including ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO), and proved their capacity to completely oxidize ammonia to nitrate. They have been named complete ammonia oxidizer (Comammox). Comammox could be subdivided into two branches of clade A and clade B based on the sequence discrepancy of ammonia monooxygenase subunit alpha (amoA), which have been widely found in natural and engineered environments such as soil (paddy soil, forest), fresh water (wetland, river, lake sediments, aquifers), wastewater and drinking water treatment plants. This paper introduces the discovery and distribution of Comammox, focuses on reviewing the research development on Comammox and prospects its research and application as a significant functional group of nitrogen cycle.